1. Field of the Invention
The present invention relates generally to electronic fuel injector systems for internal combustion engines, and more particularly, to an electronic fuel injector driver circuit for controlling electromagnetic fuel injector valves for use on internal combustion engines.
2. Description of the Related Art
With the recent interest placed on efficient use of space in automotive vehicles, automotive vehicle manufacturers have asked designers to give-up more engine compartment space for interior passenger compartment space. This is known as "cab forward" design and is quickly becoming commonplace in the automotive industry today. The cab forward design puts a premium on space in the engine compartment, while the customer puts a premium on performance and power. Styling has also played a role in the decay of engine compartment space. Lower hood lines with non-existent front grills are very common. All of these factors have led to the recent renewed interest of applying two-stroke internal combustion engine technology to the automotive vehicle.
One major hurdle in applying two-stroke internal combustion engine technology to the automotive vehicle is the air/fuel delivery into combustion chambers of the engine. The conventional two-stroke internal combustion engine has a crankcase which receives the air/fuel/oil mixture that is then transferred to the combustion chamber during the "power" stroke. This fuel delivery scenario is deemed unacceptable in automotive applications where governmental regulations are getting increasingly more stringent. Clearly, a solution must be derived whereby the air/fuel delivery and the crankcase lubrication system are separated in a manner similar to four-stroke internal combustion engine technology. Recently, a new "external-breathing-direct-fuel-injected" two-stroke internal combustion engine has been developed specifically for automotive vehicles. The engine "breathes" or receives fresh air via an external blower and fuel is injected directly into the combustion chambers during the compression portion of the power stroke.
This new fuel delivery system presents challenges in the area of fuel injection and control. New fuel injectors have been developed to meet the physical requirements of injecting pressurized fuel into pressurized cylinders, achieving proper atomization, and the like. However, these fuel injectors, in order to complete their task, must be controlled in a manner which deviates from the typical control systems present today.
In light of present day consumer demand and stringent government regulation, fuel injector system technology must continue to advance forward. Systems which provide improved performance, better fuel economy as well as reduced exhaust emissions must overcome inherent design limitations which constrain fuel injector valve response time. Primary factors affecting fuel injector valve performance are injector solenoid coil current rise and fall times. Typically, fuel injector response time has been improved by rapidly building the injector solenoid coil current until the injector valve begins to open. The fuel injector valve driver circuit then reduces the applied current to a lower `holding` value to avoid overheating the injector solenoid coil winding. Finally, current is abruptly turned `off`, and injector solenoid coil current is recirculated through the coil giving a fairly slow injector valve `close` time.
Fuel injector systems for two-stroke internal combustion engines must utilize an improved version of this control method. The fuel injector system must have the capability of being able to actuate and hold open fuel injector valves for between 200 and 2,000 microseconds which is much shorter than the 2,000 to 10,000 microseconds found in four-stroke internal combustion engines. Short actuation times require ultra-fast fuel injector valve response. As a result, there is a need in the art to provide an electronic fuel injector driver circuit which overcomes the inherent electromechanical fuel injector valve delay problem which can clearly be illustrated in the example below.
Typically, a two-stroke internal combustion engine has an operating condition which requires a five hundred (500) microsecond fuel injector valve actuation time (includes open, hold and close time). This requires that the fuel injection driver circuit produce an electrical pulse five hundred (500) microseconds long. This 500 microsecond valve actuation pulse width involves building up the injector solenoid coil to the `opening` current of approximately 6-9 amps in approximately 150 microseconds or less, sustain the `opening` current value for approximately 50 microseconds, ramp down to the `hold` value of 1-2 amps in less than 50 microseconds, sustain at the `hold` value for 250 microseconds, finally ramping down to zero, closing the injector valve. Fuel injectors developed for two-stroke internal combustion engine applications typically have an inductance of between 2-3 millihenries and a resistance of 1-2 ohms. Choosing a typical value of 2.4 mH and 1.8 ohms, injector valve time lag can be shown using Equation 1: EQU t.sub.r =(L/R)ln[1/(1-(I.sub.pk *R)/V.sub.BAT)] Equation 1
t.sub.r =opening current rise time PA1 L=fuel injector coil inductance PA1 R=fuel injector coil resistance PA1 I.sub.pk =peak or `opening` current PA1 V.sub.BAT =battery voltage
In this example, it can be shown that for such a fuel injector, t.sub.r, or the time needed for the injector solenoid coil current to rise to the level needed to open the injector valve, 310 microseconds would have elapsed. Thus, this method is too slow for two-stroke internal combustion engine applications requiring short fuel injector actuation times.